1. Field of the Invention
The present invention relates to an image encoding method, an image encoding apparatus and an image recording medium. More particularly, the present invention relates to an image encoding method, an image encoding apparatus and an image recording medium used in a system for encoding for storage purposes video signals of a moving image and recording the encoded signals in an image recording medium, such as an optical disk, magnetic disk or magnetic tape, or a system for transmitting video signals of a moving image over a transmission line.
2. Description of the Related Art
When image data is digitally recorded in magneto-optic disks, magnetic tape or the like, or when image data is transmitted via a predetermined transmission medium, the amount of data is reduced by encoding and compressing the data.
Here, a description will be given of a case in which the encoding standard of what is commonly called MPEG (Moving Picture Image Coding Experts Group) is used as an encoding standard.
In encoding by this MPEG method, one video sequence is divided into groups of pictures (GOP), for example, 15-frame units. Each frame is classified into the following three types according to a predictive method:
I picture: (Intra-frame-coded picture), intra-frame-coded picture, PA1 B picture: (Bidirectionally-predictive-coded picture), picture of frames for performing movement prediction from past or future P pictures or I pictures.
P picture: (Forward-predictive-coded picture), picture of frames for performing movement prediction from past or future P pictures or I pictures,
In such a compression method of what is commonly called the MPEG method, an image of each frame is predicted in units of macro blocks in any one of the intra-encoding, forward-predictive encoding, backward-predictive encoding, or bidirectionally predictive encoding, and a predictive error is encoded and transmitted. Since, basically, only predictive error is transmitted, it is possible to compress the amount of data to an extent greater than in a case in which image data of each frame is transmitted as it is.
FIGS. 9A and 9B show the relationship between these three types of images; FIG. 9A shows a reference relationship between I pictures and P pictures. In the figures, the image of the lattice pattern represents the I picture, and the image of slant pattern represents the P picture. Further, FIG. 9B shows a reference relationship between the B picture and the I/P pictures. A predetermined number of these pictures form a group of images sequences called a group of pictures.
FIG. 10 shows the structure of each picture. Each picture (frame) is formed of a plurality of units called slices, which are divided horizontally. Further, each slice is formed of a predetermined number of macro blocks. Each macro block is formed of luminance data formed of 4 blocks of 8 pixels.times.8 pixels, and color-difference data formed of each block of 8 pixels.times.8 pixels.
Meanwhile, in a pay television system, scrambling is performed to limit the listening and viewing by other than subscribers. This scrambling is based on a system such that an electronic key is given for receiving equipment of a subscriber, a scheme, for example, white and black is reversed at a predetermined cycle, is provided in a blanking period of broadcasting signals, and a normal image is obtained by this key.
Another known scrambling method is such that the phase of a burst is randomly reversed. According to this technology, in a scrambled state, an image becomes extremely poor because color differences change one after another though the shapes can still be distinguished. Further, since there is no deterioration of images due to scrambling, high-quality images can be transmitted in an unscrambled state.
Furthermore, there has been a demand for similar scrambling also in recording media, such as disks. More specifically, since the image quality of a video disk recorded by digital signals does not deteriorate even if copies are made repeatedly, there is a possibility that a large number of illegal copies might be supplied.
Regarding handling of keys for unscrambling, for example, a public key method is known. There are a large number of publications in respect of this public key method in addition to the patent of, for example, U.S. Pat. No. 4,200,770, and therefore, a detailed description thereof has been omitted because the method is a known technology.
Utilization of a scheme such that only an authorized disk can be reproduced by using this public key method is currently being considered.
In the past, when performing scrambling, there has been no appropriate scrambling, in particular, using an MPEG method. For example, the above-described scrambling which reverses the phase of a burst randomly is effective, but this scrambling cannot be immediately implemented because there is no blanking period in the MPEG method.
Accordingly, a scrambling method for the MPEG method having an effect similar to randomly reversing the phase of a burst has been desired.